The present invention relates to a data transmission system for transmitting data over telephone lines or like analog lines and, more particularly, to a jitter cancelling apparatus for carrier phase control associated with a receiver of such a data transmission system.
In a data transmission system, signals usually undergo various kinds of deterioration as typified by amplitude distortions, delay distortions, carrier frequency offsets and carrier phase jitters while being transmitted over lines. Among them, amplitude distortions and delay distortions are almost time-invariant or, if time-variant, the variation is slow enough to allow such distortions to be compensated for by so-called automatic equalizers. Carrier phase jitters, on the other hand, result in time-variant distortions and this kind of distortion has hitherto been absorbed by a phase locked loop or like feedback control system. However, since the carrier phase jitters develop in, for example, a carrier supply device of an analog transmission link and include periodic components approximating the ac (alternate current) cycle of 50 or 60 Hz of a commercial power source, the phase locked loop will fail to absorb 60 Hz jitters under a baud rate of 2400 Hz unless its equivalent quality factor is limited to less than 40. This deteriorates the Gaussian noise suppression ability of the phase locked loop.